In recent years with rising environmental awareness, soil contamination problems caused by disposal of plastic products and the global warming problem caused by the increase of carbon dioxide owing to incineration attract attention. Research and development are energetically conducted on biodegradable resins as a control measure for the former and on resins made of the raw materials derived from plants incapable of giving additional loads of carbon dioxide into atmospheric air even with incineration as a control measure for the latter. Various biodegradable resins and resins made of the raw materials derived from plants are developed for shape holders used for exhibition packaging and the like of various commodities and for containers such as food trays and beverage cups. Above all, especially polylactic acid as an aliphatic polyester, which has a glass transition point of as high as approximately 60° C. and is transparent as a biodegradable plastic material derived from plants, attracts utmost attention as a promising material.
However, in the case where polylactic acid resin is melted and subsequently cooled and solidified on a casting drum, to obtain a cast film, a lactide is discharged together with the polylactic acid resin extruded from a die, and the lactide is deposited on the casting drum, to generate defective contact spots, raising the problem of film quality irregularity. Further, the electrostatic application method employed as a method for keeping the molten film in contact with the casting drum is effective when the production line speed is low, but if the production line speed is raised, air in the entrained air stream is caught in the contact interface between the extruded resin film and the casting drum surface, to disturb the contact of the film with the casting drum, raising the problem that the productivity cannot be enhanced.
As an means for solving the problem, proposed are a technique of specifying the casting drum temperature in the case where the molten polylactic acid resin is cooled and solidified on the casting drum, a technique of melt-extruding the polylactic acid resin as a film and cooling it via a water film on the casting drum, and a technique of bringing the resin film extruded through the slit clearance of a die into contact with the moving casting surface using two or more contact means different from each other.
For example, JP 2004-299099 A describes a method for producing a polylactic acid film at a high production efficiency by keeping the film temperature on the separation surface at the time of separation from the cooling drum at 25° C. to 45° C. to inhibit the oligomer deposition on the surface of the cooling drum used for forming a film of polylactic acid resin. However, in this production method, though the oligomer deposition generated from the polylactic acid resin on the casting drum can be inhibited at a relatively low casting speed, the oligomer contamination of the electrode used in the electrostatic application device cannot be inhibited in the course of long-term production. Further in a higher casting speed region, the pulsation of edge portions cannot be controlled and, in addition, since the casting drum temperature is high, it is difficult to separate the film. Accordingly the enhancement of productivity as achieved by enhancing the production speed cannot be observed.
JP 2000-108542 A describes a production method of melt-extruding polylactic acid resin as a film and cooling and casting the film via a water film on a casting drum. However, according to that production method, at the time of production in a low speed range, a defect-less film can be obtained while the oligomer deposition generated from the polylactic acid resin on the casting drum can be inhibited, but when the casting speed is in a high speed range, the pulsation at the edge portions cannot be controlled though the entrained air stream can be inhibited. Accordingly no enhancement of productivity can be observed.
JP 2009-39890 A discloses a method for producing a polyester sheet by applying spot air masses to the edge portions and further electrostatically bringing the edge portions into contact with a cooling drum when a wire electrode is used to bring the sheet into contact the cooling drum over the entire width.
JP 2000-202842 A discloses a film production method in which two or more contact means different from each other are applied. JP 2009-62410 A discloses a polylactic acid sheet production method, in which a sheet is brought into contact with a drum or the like while being controlled at a specific temperature.
It could therefore be helpful to provide a method for producing an aliphatic polyester film that is small in thickness irregularity, excellent in flatness, small in the differences of mechanical properties and optical properties between the machine direction and the transverse direction and, in addition, allows casting at higher speeds.